PHNOM PENH, Cambodia (AP) _ Two statue fragments from the ancient Angkor temple complex, seized by police from an antique store in Thailand six years ago, were returned to Cambodia on Monday.

The pieces, uncrated at Cambodia's National Museum, were the first of a total of 13 artifacts being returned from Thailand. They are the upper half of a stone human head, thought to be from the late 12th century, and the head of a Hindu demon from the 10th century.

Smuggling of Cambodian artworks is a serious problem, and thieves and collectors especially prize artifacts from Angkor, ancient capital of one of the world's greatest civilizations.

Thailand had stalled in returning the objects, claiming Cambodia had not presented proof of ownership.

Negotiations with Thailand over the return of other artifacts are ongoing, said Sebastien Cavalier, a spokesman for the U.N. Educational, Scientific and Cultural Organization.